The present invention relates to a process for steam dewatering of high moisture organic solid materials, in particular, coal in its early stages of formation such as peat, brown coal, lignite and subbituminous coal.
In this specification such high moisture organic solid materials will be deemed to be included in the term "brown coal".
Since brown coal is porous and contains a large quantity of water in its capillaries, its utilization has been limited to the areas around the mine sites despite the existence of huge reserves. To use brown coal in areas remote from the mine sites, it is desirable to reduce its moisture (and therefore weight) and thereby improve the economy of transporting it.
However, the ordinary evaporative drying methods are not suitable for brown coal because it consumes a large amount of latent heat for evaporation, and the dried product is dusty and dangerous because of the possibility of spontaneous ignition or a dust explosion.
The only known prior art method suitable for brown coal is steam dewatering which was orginially disclosed by Fleissner (U.S. Pat. Nos. 1,632,829 and 1,679,078).
The original concept of steam dewatering consisted of first heating brown coal in pressurized saturated steam so as to prevent the evaporation of the moisture from coal and then reducing the steam pressure thereby making the moisture evaporate.
Later, it was discovered that a large amount of water was released from the coal without evaporation during the heating stage, because of the destruction of the colloidal structural of coal, with the result that heat consumption is small and the coal quality is upgraded during heating.
The use of hot water instead of saturated steam was disclosed by U.S. Pat. No. 3,552,031, but this process is unsatisfactory because during the depressurizing state the moisture can no longer decrease by evaporation, but in addition it even increases the moisture because the coal reabsorbs water when cooled.
An industrial process of steam dewatering was disclosed by Kretchmer (Austrian Pat. No. 190,490) employing a number of autoclaves containing brown coal and the same number of condensate tanks attached to the autoclaves (the autoclaves and the condensate tanks being connected in pairs) to receive and store the hot water (a mixture of steam condensate and moisture removed from coal) generated at each of the autoclaves.
According to this Austrian patent, when a pair consisting of an autoclave and a condensate tank conducts the depressurizing stage, there is prepared another pair of an autoclave and a condensate tank which is in an earlier portion of the heating stage, so that steam and/or hot water exhausted from the depressurizing pair flows into the heating pair and the heat is utilized to preheat the coal in the heating pair.
Another industrial process of steam dewatering is disclosed by Schuster (U.S. Pat. No. 3,007,254), but this process is uneconomical because it requires a number of accumulators in addition to the pairs of autoclaves and condensate tanks, and instead of a direct heat exchange between autoclave pairs, the exhausted steam and hot water from the depressurizing pair are first stored in accumulators and later flowed into the preheating pair.
Both of the processes disclosed by Kretchmer and Schuster can be termed a "closed heating process", because heat recovery is carried out only from the depressurizing stage and not from the heating state. This means that autoclave pairs are always closed through the entire heating stage except for the final discharge of waste water to the outside of the system.
Although the steam dewatering process disclosed by Kretchmer is the most successful prior art system, it has the following drawbacks because it is a "closed heating process":
(1) The heating of the coal is insufficient and some portion of the hot water tends to remain among the coal particles and be reabsorbed during depressurization, especially when the particle size is small, because the autoclave pairs are closed during the heating stage and steam does not readily flow through the coal beds in them. PA1 (2) A large number of condensate tanks are required, because all of the waste heat consumed during the heating stage must be kept stored mainly in the form of hot water which is exhausted only after the beginning of the depressurizing stage as the preheating medium for other autoclave pairs during the earlier portion of heating stage. PA1 (3) The depressurizing time is long, because a large amount of heat must be recovered, in spite of the fact that the faster the depressurization, the larger is the moisture evaporation during depressurization. PA1 (4) The depressurizing time cannot be shortened, also because it should be equal to the time of the earlier portion of the heating stage to preheat the coal by the waste heat recovered therefrom. PA1 (5) The average processing capacity per autoclave is small and the equipment cost becomes high, because the single batch processing time is unnecessarily long. PA1 (6) More than two of the autoclaves forming a heat exchange group are connected with an external steam source simultaneously for a certain period, wherein a greater amount of steam tends to flow into the downstream autoclave having a lower temperature and pressure which started in the heating stage later, and the steam flow drops in the last period of heating which is most critical for dewatering, because fresh steam must be directly supplied from the external source through all of the remaining later portion of the heating stage after the heat recovery from the depressurization stage. PA1 (7) The steam temperature does not become high enough in comparison with the adapted pressure at the end of the heating stage, because the partial steam pressure is lowered by the presence of the noncondensible gas decomposed from the coal by the heat. There is a known art method to draw off the gas at the final portion of the heating stage, but it is incomplete, dangerous and accompanied by a considerable steam loss. PA1 (1) an atmospheric pressure stage to unload the coal dewatered and to load the coal to be dewatered. PA1 (2) a heating stage to heat and dewater the loaded coal and PA1 (3) a depressurizing stage to lower the interior pressure for the unloading of coal. PA1 (1) closed heating by recovered steam is carried out at the initial steaming step PA1 (2) closed heating by fresh steam is carried out at the first steaming step, and PA1 (3) ventilating heating by fresh steam is carried out at the second steaming step. PA1 (1) closed heating by recovered steam is carried out at the initial steaming step and at the earliest of the intermediate steaming steps, PA1 (2) ventilating heating by recovered steam is carried out at the remaining intermediate steaming steps, and PA1 (3) ventilating heating by fresh steam is carried out at the first and the second heating steps
The process step which comprises the heat recovery from the heating stage can be called the "ventilating heating process", because steam flows through the autoclave during the heating stage and waste heat is recovered therefrom simultaneously.
The concept of the "ventilating heating process" has already been disclosed by some of the present inventors and other people in Japanese Patent Provisional Publication No. 58-142987 laid open on Aug. 25, 1983. However, the disclosed process is not sufficient to eliminate the drawbacks of the conventional closed heating process, because it discloses only the process of eliminating the problems of the residual inner-particle water, wherein brown coal is enclosed in a plurality of pressure vessels, superheated steam is fed into the first pressure vessel to dewater the coal, the saturated steam or nearly-saturated steam is discharged from the vessel, and fed into the second pressure vessel or vessels to effect saturated steam dewatering of the coal therein. The above disclosure does not teach which period of the heating stage the waste heat should be recovered from nor passed to, and from which autoclave to which autoclave the heat should be recovered. Since then, the disclosure does not indicate a way to utilize the "ventilating heating" to solve the problems of the "closed heating process".
It is an object of the present invention to provide a steam dewatering process for brown coal with high dewatering performance and low equipment cost, by eliminating the aforementioned drawbacks of the conventional closed heating process.
Another object of the present invention is to provide a steam dewatering process for brown coal, wherein ventilating heating is carried out effectively.